Astronomy, Public

Gravitational Waves: Meet the Experts

On Saturday 20th February 2021, we hosted a “Meet the Experts” session as part of Cardiff Science Festival. You can watch a recording of the event below.


We didn’t get to all the questions live, but the panel kindly answered them by text. Below is a list of the questions and answers, with links to those answered live.

  • Does a cosmic event trigger one pulse or a series of pulses over a longish period?
    • Fabio Antonini: It depends on the type of astrophysical source. Supernovae and head-on collisions between black holes can produce one single powerful burst (or pulse). However, LIGO has so far only detected binaries that gently inspiral towards each other before merging. These binaries produce a continuous signal that can last from fraction of seconds for black holes to hundreds of seconds for neutron stars.
  • Students ask why GW scale with distance to source, not inverse square. Is there a simple explanation please?
    • This question was answered live by Hong – watch the answer
  • If Einstein discovered that gravity isn’t a force, then why is it considered to be one of the four fundamental forces?
  • Can you tell how long the waves you detected have been travelling through space
    • Bernard Schutz: Yes, there is information in the signal we get from merging binary systems that contains their distance! This is very unusual in astronomy!
  • Why is the detector underground?
    • Keiko Kokeyama: The KAGRA detector is built underground to avoid ground motions. Grounds are actially moving all the time with or without earthquakes. These ground motions are so small and slow that we don’t feel.
      Because gravitational wave detectors must be very very sensitive, we want the grounds to be very quiet. If we go underground deeper, it is quieter. That’s why the KAGRA detector is built underground.
      The LIGO detectors are built overground. But instead, they have special ground motion isolation machines.
  • Did I see that the LIGO mirrors are made of sapphire? Why is this used and what percentage of the intensity of the original beam is detected?
    • Keiko Kokeyama: LIGO detectors use fused silica. Fused silica is very high quality quartz. To make the detector very very sensitive, the material property of fused silica is good (making less noise) in a room temperature.
    • Sapphire mirros are used in KAGRA. Sapphire has very good property in the cryogenic temperature (-250 deg C!). Their plan is to refrigerate the mirrors to lower so-called “thermal noise.”
  • Will the Kagra detector be able to detect earthquakes as it is located in Japan?
    • Keiko Kokeyama: All the detectors anywhere in the world can see earthquakes because they are very very sensitive!
  • What are the range of wavelengths (max/min) these gravitational wave detectors can detect?
    • Keiko Kokeyama: LIGO, VIRGO, KAGRA detectors are designed to be sensitive to the gravitational waves at roughly 100 Hz.
  • Can you calculate the corresponding wavelength assuming the speed of the gravitational wave equals to the speed of light, 3×10^8 m/s?
    • Chris North: Yes, just as with electromagnetic waves
  • Dr Aldo said they think space time is quantum in nature – so are you saying space time is in multiple states at the same time – both there and not there at the same time?
    • Aldo Ejlli: We think that space and time are quantised at the level of the Planck scale 10-35 m in space, and 10-43 s in time and they should be at the same time. According to the Holographic principle, and Black Hole entropy, we could look for signature of this quantisation using two co-located Michelson Interferometers.
  • If there are lots of gravitational waves, Do you think it could affect humans?
    • Keiko Kokeyama: I don’t think so. Gravitational waves stretch and squeeze the space itself (more precisely, the space-time), but they are extremely small. The effect of this stretch-and-squeeze of space is typically only 10-21 meter per 1 meter. It is a ratio between a hydrogen atom size and the Earth-Sun distance!
  • What range of objects are currently detectable (smallest to largest masses, for example).
    • Hong Qi: For the current sensitivity of the detectors, we can measure black holes as heavy as several hundred times the weight of the Sun, and neutron stars as light as a couple of times the weight of the Sun. The heaviest we have measured is 160 times the Sun. The lightest neutron star is a little heavier than our Sun.
  • What value for the Hubble constant was obtained? How can this be determined using gravitational waves?
    • Hong Qi: We measured a value of 70 km/s/Mpc (uncertain by about 10 km/s/Mpc) with GW170817 binary neutron star merger. Gravitational wave can measure distances directly and does not rely on things like distance ladders in electromagnetic wave observations. So we can apply Hubble’s law that needs both the distance of the source to us and the velocity of the source moving away from us to calculate Hubble constant.
  • Are there still plans to put a detector in space? If so, will Cardiff be involved?
    • Bernard Schutz: Yes – the LISA mission is being built now by the European Space Agency. It is a complicated mission, with three separate spacecraft, and ESA plans to launch it in 2034. Because the UK is a member of ESA (not affected by Brexit!), all UK universities will have access to the data. But Cardiff is already involved, and will be making contributions to the planning for data analysis before 2034.
  • Is it possible that gravity isn’t quantised so that gravitational waves don’t have gravitons (wave-particle duality)?
    • Aldo Ejlli: There is no evidence that gravity is quantised so far. There is a problem between General Relativity and quantum Mechanics at small scales. These two beautiful theories they do not agree.
  • Does gravitational waves have any practical effect on space on a large scale e.g. compressing gas clouds or ‘pushing’ massive objects?
  • If you can bounce light between mirrors 450 times to form a path from Cardiff to Berlin, what is the advantage for building a tunnel over 4km long to bounce light the same distance?
  • How are GW detectors and instruments tested and calibrated if they are required to be so sensitive? Presumably it’s not possible to generate a ‘test’ GW.
  • Students also ask; How near would we need to be to a merger in order to feel it? (James Salmon)
  • How does the gw polarization will differ if we consider scalar-tensor theoies like brans-dicke one?
    • Bernard Schutz: Yes, Brans-Dicke theory adds a scalar field to general relativity, so there is one extra polarisation. The stretching is equivalent to expanding and contracting a circle rather than forming an ellipse out of a circle. It is transverse to the direction the wave moves in, just like the standard Einstein polarisations. When we analyse our data with 3 or more detectors, we have enough information to detect such a polarisation. So far we have not seen evidence for it.
  • Are there any new types “standard candles” observable now with gravitational detectors (beyond supernova 1a is with light brightness)?
    • Bernard Schutz: The merging binaries that we detect in GWs are standard candles themselves – we call them standard sirens, using the analogy with sound. We can tell the distance to each such event from our data.
  • Why is the amplitude of GW so small?
    • Keiko Kokeyama: It is because the interaction between the gravity and mass is extremely small.
  • How fast do the gravitational waves travel?
    • Keiko Kokeyama: At the speed of light according to General Relativity.
  • So what happens to the time component of “space-time” during gw formation from bh-bh in spiral and ringdown … as space component squeezes?
    • Bernard Schutz: The separation of time and space depends on what observer you use. But for each observer, the wave that passes stretches space between two objects (our mirrors) in terms of their separation at a give proper time. Time is certainly affected by the merger, and affects the way the merger happens, but we are so far away that the wave can always be described as a stretching of space.